Integrated into release/v3.8.12. Salvaged the emitHookBlocking payload-chaining fix from the now-closed plugins-v4 branch (#3221) and adapted it to the shipped release hooks.ts: each blocking handler now sees the body/metadata as mutated by previous handlers. TDD regression test included (RED before, GREEN after); existing plugins-hooks suites green (19+5), typecheck + lint clean.
